The present invention relates generally to deep water offshore drilling utilizing floating rigs, and more particularly, to a method for assembling, transporting and installing a drilling assembly, including a drivepipe.
In deep water drilling operations, shallow water flow (SWF) hazards have become increasingly troublesome. SWF derives its name from the phenomena of a flow, emanating from a subsurface and overpressurized zone, back to the seafloor. An overpressurized subsurface zone is formed naturally when an impermeable seal is formed over sandy settlements by rapid deposition of silty material. As the silty material is deposited over the sealed, sandy aquifer, the trapped water in the sandy settlement is unable to escape. Over time, the pressure increases in the sandy aquifer until the pressure developed is equal to or greater than the hydrostatic pressure at the depth of water at the location of the sandy aquifer. A shallow water flow occurs when the impermeable seal of silty material is penetrated to release the overpressure within the sandy aquifer. In some cases, the pressures are high enough to cause powerful flows of water and sand into the well bore. Waterflows destabilize the wellbore through erosion to collapse and in some cases damage the well bore and others adjacent thereto. Shallow waterflow hazards have been encountered in many areas of the world and continue to be a problem in deepwater drilling operations.
One solution for avoiding shallow waterflow hazards is to use a drivepipe. The drivepipe is driven into the formation past the high pressure sandy aquifer. The drivepipe becomes the casing for the well bore through which subsequent drilling operations may be conducted. Since the drivepipe is driven into the formation, the soil is compressed and compacted in the immediate vicinity of the drivepipe. Compacted soil seals the drivepipe in the formation to prevent shallow water flow around the drivepipe.
Drivepipes are usually 30 to 36 inches in diameter, having a wall one inch thick, although in some instances, the drivepipe can be 42 inches, or larger, in diameter, with a two inch wall thickness. Drivepipes are typically 350 to 450 feet in length for shallow water drilling operations if driven from the surface. In drilling operations, it has been found that a drivepipe can not penetrate beyond a certain amount, usually around the 450 feet length, because at that length, the resistance caused by skin friction becomes greater than the force of gravity and the force applied from the surface by conventional hammer means. The drivepipe will reach a point of refusal and any further force applied to the uppermost section of the drivepipe will cause yielding of the pipe material and any further driving efforts must be discontinued.
In deep water drilling operations, drivepipes having lengths of 1000 feet or more are sometimes required to mitigate shallow water flow hazards. Therefore, auxiliary means for driving drivepipes are necessary to augment the gravitational forces acting on the drivepipes to increase the depth of penetration of the drivepipes. Presently, drivepipes of this length are driven into the seafloor only by the force of gravity. At greater water depths, the application of force to the tops of the drivepipes is impractical due to the wasted energy absorption by the longer lengths. Although proven that any force applied is more effective if applied at the bottoms of the drivepipes, few methods exist for this application and are limited by the long umbilical required to supply electrical or hydraulic forces to the hammers. Some drivepipes are equipped with a drilling assembly and/or a drive assembly. The drill string continues below the running tool and extends down the entire length of the interior of the drivepipe. The lower end of the drill string assembly terminates with a jet sub or downhole motor connected to a stabilized drill bit. In alternative drivepipes, a drill bit is located at the mouth or lower opening of the drivepipe, and is driven by the motor to function as a jetting assembly to drill a hole approximately the size of the inner diameter of the drivepipe.
In an offshore deep water drilling installation, a drivepipe is typically installed by assembling the drivepipe one section at a time as the sections are hung from a floating drilling vessel. Drivepipes are assembled from various sections of pipe and fitted with the necessary driving mechanisms on the floating drilling vessel at the production site. Lengthy drivepipes having complicated driving mechanism require a good deal of time to assemble on the floating drilling vessel. Since the cost of a drilling project is directly connected to the amount of time that a floating drilling vessel must be dedicated to the particular drilling project, cost reductions may be obtained by accelerating the drilling process. The time required to assemble a drivepipe and fit the drivepipe with the necessary driving mechanisms has been identified as a significant portion of the overall project duration parameter. Therefore, there is a need for a method for assembling the drivepipe and fitting the drivepipe with the necessary driving mechanisms in such a way that the time required for the floating drilling vessel to be at the drilling site is reduced.
The present invention is a method and apparatus which reduces the time required for a floating drilling vessel to be at the production site because the vessel is not required during drivepipe assembly. In particular, the present invention is a process for assembling a drivepipe in its entirety at a shoreline assembly station and transporting the assembled drivepipe to the drilling vessel. Once delivered, the drivepipe is made up to a running string and lowered to the seabed. This invention obviates the requirement that the floating drilling vessel be used during the assembly process.
According to one aspect of the invention, there is provided a method for assembling and delivering a drivepipe to a drill site, the method comprising: assembling the drivepipe at a shoreline assembly station; attaching floatation devices to the drivepipe and floating the drivepipe in the ocean; towing the drivepipe with tow vessels to a drill site; delivering the drivepipe in a horizontal orientation to a drilling vessel at the drill site; attaching a upper end of the drivepipe to the drilling vessel and detaching the tow vessels from the drivepipe; pivoting the drivepipe about its point of attachment to the drilling vessel until the drivepipe is vertically oriented; deactivating the buoyancy function of the floatation devices attached to the drivepipe; removing the floatation devices from the drivepipe; equipping the drivepipe with a driving mechanism; and lowering the drivepipe with a running string from the drilling vessel to the sea bed.
According to a further aspect of the invention, there is provided a process for assembling a drive pipe for use at an offshore drill site, the process comprising: assembling the drive pipe at a shoreline assembly station; floating the drive pipe in the ocean; transporting the floating drive pipe from the shoreline assembly station to the offshore drill site; suspending the floating drive pipe from a drilling vessel at the offshore drill site; and deploying the drive pipe to the ocean floor.
According to still another aspect of the invention, there is provided a process for assembling a drive pipe for use at an offshore drill site, the process comprising: assembling the drive pipe at a shoreline assembly station, wherein the assembling comprises fitting pipe sections end to end; floating the drive pipe in the ocean; transporting the floating drive pipe from the shoreline assembly station to the offshore drill site, wherein the transporting comprises towing the drive pipe through the ocean, wherein the drive pipe is submerged in the ocean in an approximately neutrally buoyant condition during the towing; suspending the floating drive pipe from a drilling vessel at the offshore drill site, wherein the suspending comprises coupling an upper end of the drive pipe to the drilling vessel and sinking the drive pipe in the ocean, whereby a substantial portion of the weight of the drive pipe is supported by the drilling vessel; deploying the drive pipe to the ocean floor, wherein the deploying comprises attaching the upper end of the drive pipe to a drill string and lowering the drive pipe to the ocean floor with the drill string.
According to another aspect of the invention, there is provided a process for reducing the amount of time required for a drill vessel to assemble and deploy a drive pipe at an offshore drill site, the process comprising: delivering an assembled drive pipe to the drill vessel at the offshore drill site; suspending the drive pipe from the drilling vessel; and deploying the drive pipe to the ocean floor.